Thermistors basically composed of Mn oxides and Co oxides have been widely used. These thermistors are generally composed of Mn-Co, Mn-Co-Cu, Mn-Co-Ni or Mn-Co-Ni-Cu oxide systems and have been used as general-purpose disc type thermistors, typically for temperature compensation. Such thermistors are typified by their specific resistance ranging from 10-odd .OMEGA..multidot.cm to 100-odd K.OMEGA..multidot.cm and have been applied to uses mostly in the temperature range of from -40.degree. C. to 150.degree. C. Recently, these thermistors have come to be used increasingly as temperature sensors, and request is growing for thermistor sensors which can be used at higher temperatures.
As the first stage, the thermistor sensors that can stand use at high temperatures up to 300.degree. C. have been required for use in temperature control of solar systems or oil combustion devices. To meet such requirement, studies have been made on thermistor materials having higher specific resistance than the conventional Co-Mn oxide-based materials, and consequently, there have been developed and put to commercial use an Mn-Ni-Al system oxide semiconductor (Japanese Patent Laid-Open No. 95603/82) and Mn-Ni-Cr-Zr system oxide semiconductor (U.S. Pat. No. 4,324,702), the latter having been proposed by the present inventors.
In the aspect of sensor structure, in order to protect the resin-molded structure of conventional disc type thermistors from high-temperature ambient air, it has been proposed to encapsulate micro-thermistor elements having a size of about 500 .mu.m.times.500 .mu.m.times.300 .mu.m(t) in a glass tube or coat such thermistor elements with glass by dip coating. Bead type thermistors, like said disc type, have been also glass coated to improve heat resistance.
However, the demand for the thermistors usable at higher temperatures was not confined there; now the request is growing for the sensors that can be used at temperatures of not lower than 300.degree. C. up to 500.degree. C. or 700.degree. C. The currently available materials have the following two problems in meeting such requirement: (1) they are low in specific resistance which is one of the characteristics of thermistor materials, so that it is impossible with these materials to obtain a resistance required for operating the device at a desired high temperature; (2) the change of resistance with time in these materials at high temperatures exceeds the highest permissible level of 5% (at 500.degree. C. in 1,000 hours), and thus they lack reliability in practical use.
On the other hand, stabilized zirconia (ZrO.sub.3 -Y.sub.2 O.sub.3, ZrO.sub.2 -CaO, etc.) and Mg-Al-Cr-Fe oxide compositions have been developed as materials usable at high temperature of 700.degree. C. to 1,000.degree. C. However, the calcining temperature of these oxide materials should also be above 1,600.degree. C., and these materials cannot be calcined with an ordinary electric furnace (max. temp. 1,600.degree. C.). Further, even the sintered bodies of these oxide materials suffer a wide change of resistance with time at high temperatures, such change being of the order of 10% (1,000 hrs.) in the most stable ones. Thus, a further improvement of reliability has been required of these materials.
Novel materials that can overcome this problem have already been proposed in Japan, but they are still in the stage of evaluation. (Mn-Zr-Ni oxides: Japanese Patent Laid-Open No. 88305/80; (Ni.sub.X Mg.sub.Y Zn.sub.Z)Mn.sub.2 O.sub.4 spinel type: Japanese Patent Laid-Open No. 88701/82; (Ni.sub.p Co.sub.q Fe.sub.r Al.sub.s Mn.sub.t)O.sub.4 spinel type: Japanese Patent Laid-Open No. 88702/82).